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CDRIVE
Hauling 10' pipe on a Trek Shift3
Heck yeah. They're a MUCH better choice for projects that demand juice. I only suggested stacking buttons because I thought you were committed to them.
Chris
CDRIVE
Hauling 10' pipe on a Trek Shift3
The SSR would be wired like this. I X'd out the 10KΩ because I don't see its purpose but replacing the button with AA cells changes the ballgame significantly with much more available power. So if you want to leave it connected it will add only 450uA of current draw when 'DO' goes low.
Chris
Chris
CDRIVE
Hauling 10' pipe on a Trek Shift3
Then leave the 10KΩ resistor alone and hookup the SSR (when you get it) as I've drawn it.
Chris
No, it actually pulls low.
CDRIVE
Hauling 10' pipe on a Trek Shift3
Do you understand what the terms "Source & Sink" mean? Steve eluded to it but you may not have grasped to concept.
Chris
Edit: So there should be no misinterpretation here, your comparator (DO) is Open Collector logic. This means it can NOT source current. It can only sink it.
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hevans1944
Hop - AC8NS
See post #36: LED OFF, RELAY ON. See my post #38. The opto-isolator in the SSR has a 440 Ω current-limiting resistor and the SSR needs about 4 V to light-up the opto-isolator LED and turn the SSR ON. So, the OP needs to place SSR control between the collector of an NPN transistor and Vcc, drive the base of the NPN through about 2 kΩ connected from Vcc to DO, connecting the base to DO with the NPN emitter connected to common.
If the SSR is connected from DO to common to turn on when the LED is OFF, there will not be enough current through the 10 kΩ resistor to provide 4 V across the SSR control input. If the 10 kΩ resistor is paralleled with a low enough resistance to turn on the SSR when DO is high, then too much current flows when DO is low. A transistor (2N3904 recommended) is needed, but the base current through the 10 kΩ resistor may not bias the base-emitter junction into saturated collector conduction. Best to parallel the 10 kΩ pull-up with 2 kΩ.
If the OP is gonna do all that, they might as well use the circuit in the original post! Just lower the value of the 10 kΩ "pull-up" resistor by connecting a 2 kΩ resistor from Vcc to DO and raise the voltage of BT1 from 3 V to 4.5 V.
hevans1944
Hop - AC8NS
It does output a positive current from Vcc through the on-board 10 kΩ "pull-up" resistor when the open-collector LM393 comparator goes "high" and thereby removes the path to common for the LED, turning the LED OFF. I am assuming you want the relay to actuate when that happens.
Unfortunately, in your original circuit, there was not enough base drive to the transistor and not enough voltage from the 3 V coin cell to fully activate the coil of the relay. Adding a 2 kΩ resistor from Vcc to DO in your original circuit will increase the base drive to the transistor, and increasing the battery voltage from 3 V to 4.5 V will make it possible to actuate the relay coil. You must do both of these things! While you are at it, it wouldn't hurt to add another AA cell in series to make Vcc = 6 V. That should be sufficient voltage to actuate any "5 V" relay, including the one you already have. It will also easily actuate the SSR if you choose to substitute that for the electro-mechanical relay.
CDRIVE
Hauling 10' pipe on a Trek Shift3
Perhaps, (to be more precise) I should have said the LM393's "Output Pin is Open Collector Logic" thus can not source. As Hop stated DO is sourced through the 10KΩ pullup resistor.
Regarding DO's output logic.... I thought we settled this as LED ON thus DO= low!
Chris
hevans1944
Hop - AC8NS
This is true. But see this: "it (the LED) seems to go off when the temperature goes above setting, which is when i would like the relay to activate" which was re-iterated in post #35 and confirmed by Steve in post #36: LED OFF, RELAY ON.
So I am thinking with the relay de-actuated, and the temperature below the set-point, NC contacts on the relay cause the thermistor probe to heat up from some sort of heater connected to those NC relay contacts. The LED is ON while things are warming up. When the set-point temperature is reached (or exceeded), the LED goes OFF, the relay energizes, and the NC contacts open to turn off the heater.
Piece of cake, and pretty much the way the original circuit in post #1 would work but for the wimpy 10 kΩ pull-up resistor and wimpy 3 V battery voltage. So raise Vcc to 4.5 or 6 V (three or four AA cells in series) and bridge a 2 kΩ resistor from Vcc to DO. Use a 2N3904 NPN transistor or something equivalent that will handle at least 100 mA collector current while in saturation. These small changes should guarantee that the transistor will conduct in saturation (very low voltage drop from collector to emitter) when the DO goes "high".
Hop
Even without the additional pull-up resistor, the original circuit should work with the SSR.
This (using an SSR) will give much longer battery life as well as having high voltage connections that are well away from the more voltage (which is not the case with most relays).
My vote on design goes to 3 or 4 AA cells and an SSR. Oh, and the SSR may have a visual indication it is on (via its own LED), which is another advantage.
This (using an SSR) will give much longer battery life as well as having high voltage connections that are well away from the more voltage (which is not the case with most relays).
My vote on design goes to 3 or 4 AA cells and an SSR. Oh, and the SSR may have a visual indication it is on (via its own LED), which is another advantage.
hevans1944
Hop - AC8NS
You don't need to know much to get your circuit working. The SSR will work "better" than a relay.
But while you are waiting for it to arrive, why not try adding an external 2000 ohm resistor from Vcc to DO to guarantee the transistor will be driven fully on (into saturation) when DO goes high (and the LED goes out)? And be sure to raise the supply voltage (Vcc) from 3 V to 4.5 V (three AA in series) or 6 V (four AA in series). And you can make sure your transistor is working by listening for the relay to "click" when the temperature of the thermistor sensor rises above your set-point temperature. Just "flick your BIC" and briefly heat the thermistor with the flame. Maybe connect a circuit to the relay contacts to make sure they open and close as expected.
hevans1944
Hop - AC8NS
With the SSR in the transistor collector circuit, there should be no problem lighting up an LED that might be present on the SSR. Nice feature, that.
hevans1944
Hop - AC8NS
Yes, the transistor is necessary to provide both operating voltage and current to the low-impedance (440 Ω) control input terminals of the SSR. The existing 10 kΩ internal "pull-up" resistor connected between Vcc and DO will act as a voltage divider to the 440 Ω input impedance of the SSR, dropping the voltage applied to the control terminals of the SSR below what is necessary to reliably turn the SSR on and limiting the current that can be supplied to the SSR's internal LED/optical isolator.
Depending on how "sensitive" the control input to the SSR happens to be, it might operate okay connected between DO and common, but a transistor solves any problem by driving the SSR control input terminals between Vcc and the the "virtual" ground provided by the NPN transistor when it is conducting in saturation, i.e., when DO goes high. So, sure, try it without the transistor. If it works, it works. If SSR triggering is unreliable, add the transistor and make sure it goes into saturation by bridging the 2000 ohm resistor between Vcc and DO.
This is NOT rocket science.
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